The control system of the Titan liquid rocket engines is basically a hydraulic computer. One valve in particular integrates a number of functions into a complex control system. Its response time is good enough for rocket control, where anything other than combustion instability can be managed with a ~250mS response time. Combustion instability you manage a.)through designing to avoid it, and b.)by wrapping a "burn wire" around your thrust chamber so that you can tell if there's been burn through or worse, and close any remaining propellant valves to limit the damage.
I think the Titan engine is one of the finest examples of Victorian technology finished a few years too late for either Victoria or Edward.
Like analog computers, it'd cost a lot more to make a general purpose hydraulic computer than a bunch of specialized ones for specific tasks or with limited programmability. The program gets built in.
Emulsion photography can be implemented in really low tech, and look at all the applications that's got. Even without optics you have contact printing. Document and drawn image reproduction, light recording (e.g. record the trail of a match over the paper's surface), event timing, etc.
Add flat mirrors and a tent then you can make images with a camera obscura. Add the concepts of pressure and heat sensitive papers and the applications grow. You could even play tricks on local high tech civilizations that have gone digital. Do they still remember to check for latent images on paper? Or do they just do cryptanalysis on the visible part of the message because that's what's easy and obvious to them?
When I worked in a calibration lab, we had several hand-made wooden hygroscopes that we used as checks on our higher-tech humidity instrumentation. They allowed us to catch errors in the "modern" instrumentation several times in the year I was there. A set of old breaks in the bones of one of my hands let me catch errors in our electric barometer a couple of times, which led to getting special dispensation to allow a real mercury barometer in the lab. We needed special permission because of the "environmental hazard" of the mercury, never mind that the perfectly natural ground outside the back door was lousy with cinnabar (when approval for mercury was slow in coming, I suggested we get our own off the rocks. Our manager didn't approve of the idea...once he could stop laughing.)
When using retro-tech in games, I often like to have details about operational compromises that are made with the technology so that it doesn't just become a new name for the same thing. Displays that have to be used in dark rooms as the lighting is too dim to use in direct light, oars and/or sails for tacking with rotor ships (the real ships like the Baden Baden at the start of the thread had to turn off the rotors and use auxiliary engines to tack), etc.